FIG. 1 shows a conventional approach for terahertz spectroscopy and/or imaging of a bead or particle. A terahertz emitter 10 emits a pulse of radiation 20 having a frequency in the terahertz range (e.g., 0.2-100 THz), and the reflected radiation 22-26 is detected by a terahertz detector 30. In time-of-flight spectroscopy and/or imaging, a first reflection 22 from the outer surface of a coated bead 40 is detected first by the terahertz detector 30. However, part of the radiation pulse 20 passes into the outer coating 42 of the coated bead 40 on a sample mounting substrate 50 having a completely planar (e.g., horizontal) upper surface. A second reflection 24 from the outer surface of the core 44 of the coated bead 40 is detected by the terahertz detector 30 after the first reflection 22. The difference in time that the terahertz detector 30 detects the first and second reflections 22 and 24 provides information relating to the thickness of the outer coating 42. Repeating the radiation pulse emission and reflection detection at a number of different locations on the bead and/or angles between the emitter 10 and the sample mounting substrate 50 provides information relating to the uniformity of the outer coating 42.
However, reflections 26 from the completely planar upper surface of the sample mounting substrate 50 often have a time-of-flight similar to reflections 24 from the core 44 (or other layer below the surface) of the bead 40. Therefore, reflections 26 can interfere with reflections 24 from layers below the surface of the bead 40 and lead to difficulties obtaining useful or reliable information about the coating layer on a bead or particle, and sometimes can result in complete failure.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.